Valve Itch phono

[taniknight]

Salas and others,

I'm collecting parts for the Itch (rev1 as in post #31 here) and I'd like to know which version of the HV shunt reg is recommended. In the SSHV thread the evolution/update link takes me to this.

Should I collect parts for this schematic or are there better options/updates to try?

I noted some changes for higher than 300v applications... what about a nominally 300v application such as the Itch? Would you recommend the higher voltage PNP noted in the linked post?

Regards,
Tani.

[Salas]

No looking further, that link leads you to the right one. Here is an example I made once for giving +HV & heaters juice to a tubes phono, taking the place of its stock tubes regulator box. Has the perfboard build shown in the link. 320VDCin will suffice for 300VDCout. Use 1N/UF4007 1000V PIV diodes in the rectifier bridge.

[spopepro]

Salas, What is the reasoning behind using one tube as input and one as load, as opposed to using one tube per channel?

[Salas]

Mu follower. Gives low Z output point, bootstrapped constant current source, insulates from PSU rail a bit. Upper valve has heater's lift, so can't be the same envelope.

[spopepro]

Does it make a big difference? I've seen other designs where the cathode is lifted, but using 1/4 of B+ for the heater in a dual triode where one cathode is +4 and the other is something like +200. Just curious. I really don't know.

[Salas]

Mu follower is totem and with this B+ they can't share some middle ground same heater potential lift safely given their max spec. On the edge. So I went each envelope to its own ''floor''.

[taniknight]

I'm having trouble sourcing a suitable transformer for the HV shunt. I pretty much need a 240v primary / 240v secondary as I'm aiming for the 300vdc reg output, and the original power supply sketch in this thread calls for 100VA.

A few questions:

- 100VA for the HV transformer seems large compared to what I am able to commonly find available. Is there a specific reason for this rating or can I safely go smaller without too greater sacrifice?

- Some power supply designs I've seen achieve low voltage for heaters and high voltage for B+ by using a step down Tx, say 12vac secondary, then a similar Tx wired in reverse to step the voltage back up. Would such a scheme work here (or anywhere)?

- Two 9vac transformers have been specified to supply the LM317 heater regs. Are the two needed as the regs don't share a common ground reference, or could 1 Tx be used so long as there are two rectifiers?

I can source cheap 96VA multiple output (6-12-18-24-30 Vac) transformers in metal enclosures with fuse and power lead. Could I use them in the configuration shown in the attached diagram?

Your help with the power supply is greatly appreciated as it is this item that is really slowing progress right now.

Regards,
Tani.

[Salas]

What you saw it was a description of how that PSU was made with transformers stuff that were available and not strictly a specification. Anything 30VA will be more than twice what a single 40mA CCS setting shunt reg will consume at those voltages. About transformer tricks I don't know much to guide you. Looks logical your plan, but I prefer others who have done such may comment. I just say use whatever easily available that will give you the proper voltages and currents. Use two 220K resistors in that schematic for 1/2B+ heaters lift in the newer phono circuit values changes. Fuse to the transformer's rating and be careful with high voltages.

[The Space Egg Corp]

Originally Posted by taniknight

I'm having trouble sourcing a suitable transformer for the HV shunt. I pretty much need a 240v primary / 240v secondary as I'm aiming for the 300vdc reg output, and the original power supply sketch in this thread calls for 100VA.

A few questions:

- 100VA for the HV transformer seems large compared to what I am able to commonly find available. Is there a specific reason for this rating or can I safely go smaller without too greater sacrifice?

- Some power supply designs I've seen achieve low voltage for heaters and high voltage for B+ by using a step down Tx, say 12vac secondary, then a similar Tx wired in reverse to step the voltage back up. Would such a scheme work here (or anywhere)?

- Two 9vac transformers have been specified to supply the LM317 heater regs. Are the two needed as the regs don't share a common ground reference, or could 1 Tx be used so long as there are two rectifiers?

I can source cheap 96VA multiple output (6-12-18-24-30 Vac) transformers in metal enclosures with fuse and power lead. Could I use them in the configuration shown in the attached diagram?

Your help with the power supply is greatly appreciated as it is this item that is really slowing progress right now.

Regards,
Tani.

Hi Tani

I have used 'back-back' transformers for valve projects and they work fine to get your HT.

To judge the final output voltage can be tricky, but more is generaly best, and OK for regulated circuits.

Think of the primary/secondary 'ratios' ie 240v/12v = 20:1
Then if you add a reversed 12v/240v = 1:20, you get back to 240v again.
If you calculate the ratios, you can work it out for any volt combination.

In my recent valve mic project I used a 9v & 15v, as the 'ratios' worked out good for my HT volts.

The thing to watch out for is the current rating of the 2nd transformers reversed windings.
The first transformer must have high enough VA & volts for heaters (I used 9v here, as this is fine for 317 style reg (12.69v @ smoothing caps).

The reversed transformers rating is slightly different, as 1amp/12v secondary rating, means primary is only rated at 50mA/240v etc.
So don't get caught out with a low rating there.

I try to allow for my actual wall voltage (230v),
Ratio of back-back transformers,
Diode drop (hardly worth considering at HT voltages),
Volts at my smoothing caps will be approx 1.41x final transformer ouput voltage.

This method has been disputed, but unless your running a difficult load, like an arc welder, it's always worked fine for me.

I believe Salas's shunt can take quite a high input, so I guess aim half way between this and the 'Itch' HT volts. BINGO.

Hope that makes sense.

Cheers

Simon

[Salas]

20V more than Vout will be enough for the shunt reg. Calculate any losses in possible CLC or CRC pre filtering for 40mA current. Smaller transformers also sag a bit when constant current is demanded, but for 40mA should be minimal.